Caterpillars of two species of butterflies in Colorado and California aren't waiting for China and India to stop belching out so much CO2 - according to a paper in the journal Functional Ecology, they have already evolved to feed rapidly at higher and at a broader range of temperatures, just in the last 40 years, suggesting to the biologists that they did so in order to quickly to cope with a hotter, more variable climate.

This represents the first instance where recent climate change has affected physiological traits, such as the internal workings of how the body regulates feeding behavior, said Professor Joel Kingsolver at the University of North Carolina at Chapel Hill.

Caterpillars eat and grow when it's not too cold and not too hot, but when temperatures are ideal, they eat with reckless abandon and can gain up to 20 percent of their body weight in an hour. That growth determines their ability to survive, how quickly they become adult butterflies and thus their ultimate reproductive success.

Jessica Higgins, a graduate student in Kingsolver's lab who led the study, worked with fellow graduate student Heidi MacLean, Lauren Buckley, currently at the University of Washington, and Kingsolver to compare modern caterpillars to their ancestors from 40 years ago. Their results show that the two related species of Colias (sulphur) butterflies have adapted in two ways:

broadened the range of ideal feeding temperatures

shifted their optimal feeding temperature to a higher one

In their work, the researchers measured changes in climate at the two study sites and then examined changes in the caterpillars feeding rates using current and historical data from the 1970s, collected by Kingsolver's graduate adviser Ward Watt.

Although they found little change in the average air temperature at both study sites, they noticed that the frequency of hot temperatures – that is, temperatures that exceeded 82 degrees Fahrenheit --increased two-fold in Colorado and four-fold in California over the past 40 years.

In response to these temperature fluctuations, modern caterpillars in Colorado ate faster at higher temperatures than their 1970s counterparts. In California, the modern caterpillars ate faster at both high and low temperatures than did their ancestors, but their optimal feeding temperatures did not change.

Historical comparison of larval feeding rate and temperature density during the growth season for Colias eriphyle and Colias eurytheme. The solid line designates data from the past, and the dashed line is current data. The points are measured feeding rates in the Sherman&Watt (1973) experiment (mean ± SE). Relative feeding rate is calculated by standardizing the highest feeding rate for each year to one. The vertical lines indicate mean temperature. Credit and link: DOI: 10.1111/1365-2435.12218

"These two species of caterpillars adapted to the increased frequency of higher temperatures over 40 years in two different ways, but both are better suited than their ancestors to thrive in a hotter, more variable climate," said Higgins. "Our climate is changing. The thermal physiology of these species is changing, too."